Rat somatosensory cerebropontocerebellar pathways: Spatial relationships of the somatotopic map of the primary somatosensory cortex are preserved in a three-dimensional clustered pontine map

In the primary somatosensory cortex (SI), the body surface is mapped in a r elatively continuous fashion, with adjacent body regions represented in adj acent cortical domains. In contrast, somatosensory maps found in regions of the cerebellar hemispheres, which are influenced by the SI through a monos ynaptic link in the pontine nuclei, are discontinuous ("fractured") in orga nization. To elucidate this map transformation, the authors studied the org anization of the first link in the SI-cerebellar pathway, the SI-pontine pr ojection. After injecting anterograde axonal tracers into electrophysiologi cally defined parts of the SI, three-dimensional reconstruction and compute r-graphic visualization techniques were used to analyze the spatial distrib ution of labeled fibers. Several target regions in the pontine nuclei were identified for each major body representation. The labeled axons formed sha rply delineated clusters that were distributed in an inside-out, shell-like fashion. Upper lip and other perioral representations were located in a ce ntral core, whereas extremity and trunk representations were found more ext ernally. The multiple clusters suggest that the pontine nuclei contain seve ral representations of the SI map. Within each representation, the spatial relationships of the SI map are largely preserved. This corticopontine proj ection pattern is compatible with recently proposed principles for the esta blishment of subcortical topographic patterns during development. The large ly preserved spatial relationships in the pontine somatotopic map also sugg est that the transformation from an organized topography in SI to a fractur ed map in the cerebellum takes place primarily in the mossy fiber pontocere bellar projection. (C) 2000 Wiley-Liss, Inc.